Pronunciation: [stəɹˈe͡ɪdi͡ən] (IPA) 
The word "steradian" is spelled with a "st" sound at the beginning followed by "eradian." The "er" sound is pronounced like the letter "r," and the stress is on the second syllable. The IPA phonetic transcription for "steradian" is /stɛˈreɪdiən/. In physics, the steradian is a unit for measuring solid angles, with the symbol sr. The word comes from the Greek stereos, meaning "solid," and the radian, a unit for measuring angles.
A steradian is a unit of solid angle, represented by the symbol "sr". It is defined as the solid angle that covers a surface area equal to the radius squared of a sphere divided by its radius. In simpler terms, it is the solid angle formed by a cone with its apex at the sphere's center that cuts a surface area on the sphere equal to the radius squared.
The steradian is a fundamental measurement in mathematics and physics used to quantify the three-dimensional extent of a region in space as seen from a particular point. It is part of the International System of Units (SI) and is widely employed in various fields.
The steradian is analogous to the radian, which measures the extent of an angle in a plane. However, while the radian measures the proportion of a circle's circumference that an angle represents, the steradian measures the proportion of a sphere's surface area that a solid angle encompasses.
Since solid angles are essential for measuring light intensity, the steradian finds extensive application in the field of optics, particularly in the measurement of luminous intensity, as well as in radiometry and photometry. It allows for accurate calculations of lighting conditions, brightness, and luminance, making it a crucial concept in the study and analysis of light in various scientific and technical disciplines.
The word "steradian" is derived from two components: "stereos" and "radian".
The first part, "stereos", is a Greek word meaning "solid" or "three-dimensional". It is primarily used in mathematics and geometry to describe solid figures or bodies.
The second part, "radian", is a unit of measurement for angles in mathematics. It is based on the radius of a circle and is defined as the angle subtended by an arc of a circle that is equal in length to the radius. The word "radian" itself is a combination of the Latin word "radius" (meaning "ray" or "spoke") and the suffix "-an" (denoting "related to" or "pertaining to").
Therefore, "steradian" combines "stereos" and "radian" to describe a unit of solid-angle measurement in three-dimensional space.
